Heat-storing medium

ABSTRACT

Normally granulates of rare earth compounds are used as a heat-storing medium for a low-temperature range below 15 Kelvin. The material costs for rare earths are high. Further, rare earths are magnetic and thus not suitable for all applications. The present heat-storing medium for a very low temperature range is composed of a set ( 22 ) of pourable and gastight sealed hollow bodies ( 30 ). Each hollow body ( 30 ) contains a fill ( 34 ) of a low-boiling gas as a storing medium. The hollow body wall ( 32 ) is made of metal or ceramic. Thus a relatively inexpensive heat-storing medium is provided whose physical, chemical, magnetic and mechanical properties can be adapted to the respective use by corresponding material selection.

BACKGROUND

The invention relates to a heat-storing medium for a low-temperaturerange, to a regenerator for low-temperature refrigerators, and to alow-temperature refrigerator.

Low-temperature refrigerators are usually multistage gas refrigeratorswith the aid of which temperatures in the range below 15 Kelvin can begenerated. Such gas refrigerators operate according to variousprinciples, for example according to the Gifford-McMahon, the Stirlingor the pulse tube principle. Independent of the operating principles,these refrigerators have in common that they comprise, in the area of aso-called cold head between the hot side and the cold side, a volumethrough which a working fluid flows, said volume being filled with theheat-storing medium and referred to as regenerator. The working fluidflows alternately in both directions through the regenerator and servesas an intermediate storage for heat absorbed or dissipated by theworking fluid. The regenerator thus serves for thermally separating theworking fluid in the cold chamber from that in the compressor-side hotchamber. For this purpose, the regenerator must have as high a heatcapacity as possible as compared with the fluid flowing through theregenerator. While for temperatures of up to 15 Kelvin high-grade steel,bronze, lead or other metal bodies can be used, this is not possible attemperatures lying considerably below the aforementioned temperaturesince the specific heat capacity of these metals as compared with thatof helium drastically decreases as from 30 Kelvin and below, andapproaches zero in the range below 5 Kelvin. Therefore, in very lowtemperature ranges, i.e. in the range below 15 Kelvin, a fill of rareearth compounds is used as heat-storing medium in the regenerator, asis, for example, described in U.S. Pat. No. 5,186,765. A drawbackencountered when using rare earth compounds is their magnetism whichposes a problem when the compounds are employed in strong magneticfields, for example in magnetic resonance tomographs. Further, rareearth compounds are susceptible to oxidation, tend to break due to theirpartial brittleness when vibrations occur, and are expensive.

Helium and other low-boiling gases are also suitable storing media forvery low temperature ranges. For example, helium has, in the range below15 Kelvin, a high specific heat capacity with a pressure-dependentmaximum at approximately 9 Kelvin, thus in this temperature range saidheat capacity lies far above the heat capacity of metals. From DE-A-19924 184 a regenerator is known in which helium is used as a heat-storingmedium, wherein helium, like in a heat exchanger, is stationarily storedin a helically wound tube or a tube bundle in the regenerator housing.Alternatively, the regenerator housing may be filled with helium as thestoring medium, while the working fluid flows in tubes through theregenerator housing.

Tests on regenerators of such a configuration showed however that atargeted temperature of 4.2 Kelvin cannot be reached, which is due tothe high heat input from the metallic helix and tube material and thetoo small contact surface.

U.S. Pat. No. 4,359,872 describes a fill composed of helium-filled glassspheres as heat-storing medium. The wall thickness of the glass spheresmust be relatively large to present an adequate strength at the requiredinternal pressure and the low temperature.

It is an object of the invention to provide a heat-storing medium with ahigh heat capacity in a very low temperature range, a regenerator and alow-temperature refrigerator comprising a heat-storing medium with ahigh heat capacity for very low temperatures.

SUMMARY

The heat-storing medium according to the invention destined for alow-temperature range, i.e. for temperatures below 15 Kelvin, iscomposed of a set of gastight sealed hollow bodies which is permeable tothe working fluid, wherein each hollow body comprises a fill oflow-boiling gas as heat-storing medium. Low-boiling gases are gases witha boiling point below 30 Kelvin. This holds true, e.g., for the gaseshydrogen, helium and neon, and in fact to all their isotopes.Low-boiling gases have by their nature a relatively high specific heatcapacity at low temperatures and are thus well suited as storing mediumat temperatures below 30 Kelvin. Low-boiling gases are relativelyinexpensive and may be enclosed in a hollow body comprising a hollowbody wall of nonmagnetic, mechanically suited, non-oxidizing andinexpensive material. The heat-storing medium can thus be constructivelyadapted, in terms of its chemical, mechanical and magnetic properties,to any use thereof. Further, as compared with tubes and/or helices, thegastight sealed hollow bodies offer a considerably larger surface viawhich the heat exchange is effected. This considerably promotes the heattransfer.

Preferably, the storing medium is a hollow body helium fill. A heliumfill is a fill with a helium isotope, for example, with ³He or ⁴He. Thestoring medium helium has a relatively high specific heat capacity attemperatures below 15 Kelvin and is thus well suited as a storing mediumat temperatures down to the range of 2 Kelvin. Further, helium isobtainable at a low price.

Preferably, at a temperature of 4 Kelvin the helium fill has a pressureof more than 0.5 bar (7.25 psi), in particular a pressure above thecritical pressure. At a helium fill pressure of more than 0.5 bar anabsolute heat capacity is realized which allows the produced heatquantities to be stored in a relatively small regenerator. Such aregenerator is of very compact configuration as compared with metallicheat accumulators.

Preferably, the material and the wall thickness of the hollow body wallare selected such that the thermal penetration depth equals at leastonce the wall thickness. The thermal penetration depth p is representedby the following equation $\mu = \sqrt{2\frac{a}{f_{mod}}}$wherein a is the temperature conductivity of the selected hollow bodywall material at the working temperature (for example, 2 Kelvin), andf_(mod) is the modulation frequency at which the working gas cyclicallyalternately flows through the heat-storing medium. The working frequencyf_(mod) shall be assumed to amount to 1.0 to 10.0 Hz for low-temperaturerefrigerators.

The wall of the hollow body is made of metal. Metals and metal alloysoffer a good heat conductivity and good mechanical properties, whichallow a small hollow body wall thickness to be realized. The hollow bodywall can be made of copper, aluminium, silver, brass, steel or othermetals or metal alloys. Alternatively, the hollow body wall can be madeof ceramic material.

By selecting non-ferromagnetic metals as material for the hollow bodywall, a heat-storing medium can be provided which is suitable for use instrong magnetic fields, for example, for use in magnetic resonancetomographs and the like, without the need to take any further measures.

According to a preferred embodiment, each hollow body has a diameter ofless than 3.0 mm. At diameters of less than 3.0 mm a set of hollowbodies has such a large volume-specific surface that a sufficientlyrapid heat absorption or dissipation is ensured. Typical diameters rangefrom 0.2 to 0.7 mm.

Preferably, each hollow body is of approximately sphericalconfiguration. Selection of the spherical shape ensures, in the fillcomposed of hollow bodies, an approximately constant defined ratiobetween the hollow body surface, overall hollow body volume and fillmaterial volume across the overall fill material volume.

A regenerator according to the invention comprises a housing which isfilled with the heat-storing medium described above. A low-temperaturerefrigerator according to the invention comprises the aforementionedregenerator and is configured as a regenerative cycle, preferably as aGifford-McMahon, Stirling or pulse tube refrigerator, wherein helium isused as a working fluid. Thus helium is used both as a storing mediumand, separately, as a working fluid.

Still further advantages of the present invention will be appreciated tothose of ordinary skill in the art upon reading and understanding thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 shows a schematic representation of a refrigerator,

FIG. 2 shows a sectional view of a refrigerator regenerator with a fillcomposed of a set of helium-filled hollow bodies, and

FIG. 3 shows a sectional view of a helium-filled hollow body.

DETAILED DESCRIPTION

FIG. 1 schematically shows a refrigerator 10 comprising, a compressor12, a regenerator 14 and an expansion chamber 16 including a cold head.The compressor 12 as well as the regenerator 14 and the expansionchamber 16 are interconnected by lines 18,20.

The compressor 12 compresses and, if necessary, precools a workingfluid, preferably helium. Subsequently, the compressed working fluidflows through the gas line 18 and through the regenerator 14 where itdissipates heat to a heat-storing medium contained in the regenerator14. The working fluid continues to flow to the expansion chamber 16where it is allowed to expand. The cooling working fluid absorbs, inparticular via a cold surface, heat from the surroundings, and issubsequently returned through the line 20 to the regenerator 14. Whenthe working fluid flows through the regenerator 14, it absorbs heatstored in the heat-storing medium, and is returned through the line 18to the compressor 12. The regenerator 14 serves as a thermal insulationbetween the compressor 12 and the expansion chamber 16.

The refrigerator 10 can be configured as a Gifford-McMahon, Stirling orpulse tube refrigerator, it can however generally operate in any otherregenerative cycle, wherein a regenerator 14 is used for the purpose ofintermediate storage of heat in a low-temperature range. Alow-temperature range covers temperatures between 0 and 15 Kelvin.

The regenerator 14, a longitudinal section of which is shown in FIG. 2,is essentially composed of a cylindrical or oval housing 24 at whosetransverse-side housing walls 26,27 the lines 18,22 end. The regeneratorhousing 24 contains, as a heat-storing medium, a set 22 of pourable andgastight sealed hollow bodies 30, which is gas-permeable to the workingfluid. The regenerator 14 can be filled homogeneously or in layers withvarious layers of different heat-storing media.

All hollow bodies 30 have approximately the same size and are ofapproximately spherical configuration. The fill can further be composedof a mixture of hollow bodies with various diameters. The hollow bodywall 32 is made of copper or any other metal or metal alloy, and has athickness of approximately 0.2 mm or less. The diameter of a hollow body30 ranges from 0.2 to 2.0 mm, but may be larger, but not larger than 3.0mm. The hollow body 30 is gastight sealed and contains a helium fill 34.At room temperature, the helium fill 34 has a pressure of approximately200 bar (2900 psi), and at a temperature of 4 Kelvin a pressure ofseveral bars. The hollow bodies 30 containing the helium fill 34 may,for example, be produced by a manufacturing process in which drops ofthe molten hollow body wall material flow through a helium gas-filledcooling chamber. The hollow body fill can be composed of a single heliumisotope or a mixture of different helium isotopes or of isotopes ofhydrogen or neon or a mixture of the aforementioned elements. Thematerial for the hollow body wall, the modulation frequency at which theworking gas alternately flows through the regenerator, as well as thewall thickness of the hollow body must be selected such that thepenetration depth μ equals at least once the wall thickness. Thepenetration depth μ is represented by the following equation$\mu = \sqrt{2\frac{a}{f_{mod}}}$wherein a is the temperature conductivity of the selected hollow bodywall material at the working temperature (for example, 4 Kelvin), andf_(mod) is the modulation frequency at which the working gas cyclicallyalternately flows through the heat-storing medium. The working frequencyf_(mod) shall be assumed to amount, for example, to approximately 1.0 Hzfor low-temperature refrigerators.

The heat-storing medium composed of the gastight sealed andhelium-filled hollow bodies 30 has a high absolute heat storing capacityin a small volume in particular in the very low temperature range ofless than 15 Kelvin due to the high specific heat capacity of helium inthis temperature range. By selecting a suitable metal for the hollowbody wall 32, the heat-storing medium can be optimally adapted, in termsof its electrical, mechanical and chemical requirements, to any usethereof, for example, for cooling purposes in magnetic resonancetomographs nonmagnetic materials can be selected for the hollow bodywall.

Besides the helium-filled hollow bodies 30, the regenerator housing maycontain other heat-storing elements arranged in separate layers or mixedwith the helium-filled hollow bodies 30, for example heat-storingelements made of rare earth alloys.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be constructed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A heat-storing medium for a low-temperature range, comprising: a setof pourable bodies, the bodies being gastight sealed hollow bodies, eachhollow body containing a fill of a low-boiling gas as a storage medium,and having hollow body wall made of metal.
 2. The heat-storing mediumaccording to claim 1, wherein the hollow body wall is made of copper. 3.The heat-storing medium according to claim 1, wherein the material andthe wall thickness of the hollow body wall are selected such that thethermal penetration depth equals at least the wall thickness.
 4. Theheat-storing medium according to claim 1, wherein the storing medium isa fill of helium.
 5. The heat-storing medium according to claim 4,wherein the helium fill has a pressure of more than 0.5 bar at atemperature of 4 K.
 6. The heat-storing medium according to claim 4,wherein the helium fill has a pressure of approximately 200 bar at roomtemperature.
 7. The heat-storing medium according to claim 1, whereinthe wall thickness of the hollow body wall is smaller than 1.0 mm. 8.The heat-storing medium according to claim 1, wherein the hollow body isof approximately spherical configuration.
 9. The heat-storing mediumaccording to claim 8, wherein the hollow body has a diameter of lessthan 3.0 mm.
 10. The heat-storing medium for a low-temperature range,comprising: a set of pourable, gastight sealed hollow bodies, eachhollow body containing a fill of a low-boiling gas as a storing medium,and having a hollow body wall is made of ceramic material.
 11. Aregenerator for a low-temperature refrigerator, comprising: a housingfilled with the heat-storing medium according to claim
 1. 12. Alow-temperature refrigerator comprising: a regenerator according toclaim 11, and being configured as a Gifford-McMahon, Stirling, or pulsetube refrigerator, and helium gas used as a working fluid.
 13. Aregenerator for a low-temperature refrigerator, comprising: a housingfilled with the heat-storing medium according to claim
 10. 14. Aregenerator for a low temperature refrigerator comprising: a housing; aplurality of hollow, gas sealed bodies disposed in the housing, eachbody including: a body wall made of one of metal and ceramic material,which defines an interior cavity, a gas which boils at or below 30° K.disposed in the cavity.
 15. The regenerator according to claim 14wherein the gas includes at least one of helium, hydrogen, and neon. 16.The regenerator according to claim 14 wherein the body wall is thickerthan: $\sqrt{2\frac{a}{f_{mod}}}$ where a is a thermal conductivity ofthe material at a working temperature below 15° K. and f_(mod) is amodulation frequency at which a working gas alternately flows throughthe housing.
 17. The regenerator according to claim 14 wherein thematerial includes one of copper, aluminum, silver, brass, steel, andalloys thereof.
 18. The regenerator according to claim 14 wherein thebodies are less than 3 mm in diameter and a body wall thickness lessthan 0.2 mm.
 19. The regenerator according to claim 14 wherein the gasin the cavity has a pressure of at least 7.25 psi at 4° K.